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Quantum Information and Computation

Localization of quantum walks on a deterministic recursive tree

X. P. Xu

Published Mar 1, 2011• 3 citations
Publisher

Abstract

Localization of quantum walks can be characterized by the return probability, i.e., the probability for the walker returning to its original site. In this paper, we consider localization of continuous-time quantum walks in terms of return probability on a deterministic recursive tree, which is generated by adding one node and connecting it to each node of the existing tree recursively. We obtain an approximate form for the return probability using the complete eigenvalues and eigenstates of Laplace matrix of the structure. It is found that the return probability depends on the initial node of the excitation. When the walk starts at the central nodes, the return probability converges to a constant value even in the limit of infinite system, in contrast to an exponential decay of the return probability if the walk starts at the outlying nodes. We also observe a bipartite structure for the distribution of return probability, and provide theoretical interpretation for all our findings. Our results suggest that quantum walks display significant localization and well-bedded structure of return probability on heterogeneous trees.

DOI
10.26421/qic11.3-4-5

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Localization of quantum walks can be characterized by the return probability, i.e., the probability for the walker returning to its original site. In this paper, we consider localization of continuous-time quantum walks in terms of return probability on a deterministic recursive tree, which is generated by adding one node and connecting it to each node of the existing tree recursively. We obtain an approximate form for the return probability using the complete eigenvalues and eigenstates of Laplace matrix of the structure. It is found that the return probability depends on the initial node of the excitation. When the walk starts at the central nodes, the return probability converges to a constant value even in the limit of infinite system, in contrast to an exponential decay of the return probability if the walk starts at the outlying nodes. We also observe a bipartite structure for the distribution of return probability, and provide theoretical interpretation for all our findings. Our results suggest that quantum walks display significant localization and well-bedded structure of return probability on heterogeneous trees.
A return probability of random walks is one of the interesting subjects. As it is well known, the return probability strongly depends on the structure of the space where the random walker moves. On the other hand, the return probability of quantum walks, which are quantum models corresponding to random walks, has also been investigated to some extend lately. In this paper, we take care of a discrete-time three-state quantum walk on a hexagonal lattice from the view point of mathematics. The mathematical result shows a limit of the return probability when the walker starts off at the origin. The result of the limit tells us about a possibility of localization at the position and a dependence of localization on the initial state.
[[abstract]]We present a numerical study of a model of quantum walk in a periodic potential on a line. We take the simple view that different potentials have different affects on the way in which the coin state of the walker is changed. For simplicity and definiteness, we assume that the walker's coin state is unaffected at sites without the potential, and rotated in an unbiased way according to the Hadamard matrix at sites with the potential. This is the simplest and most natural model of a quantum walk in a periodic potential with two coins. Six generic cases of such quantum walks are studied numerically. It is found that, of the six cases, four cases display significant localization effect where the walker is confined in the neighborhood of the origin for a sufficiently long time. Associated with such a localization effect is the recurrence of the probability of the walker returning to the neighborhood of the origin.
We present a numerical study of a model of quantum walk in a periodic potential on a line. We take the simple view that different potentials have different affects on the way in which the coin state of the walker is changed. For simplicity and definiteness, we assume that the walker's coin state is unaffected at sites without the potential, and rotated in an unbiased way according to the Hadamard matrix at sites with the potential. This is the simplest and most natural model of a quantum walk in a periodic potential with two coins. Six generic cases of such quantum walks are studied numerically. It is found that, of the six cases, four cases display significant localization effect where the walker is confined in the neighborhood of the origin for a sufficiently long time. Associated with such a localization effect is the recurrence of the probability of the walker returning to the neighborhood of the origin.
We consider discrete time quantum random walks on a Sierpinski gasket. We study the hitting probability as the level of the fractal graph goes to infinity in terms of their localization exponents βw, total variation exponents δw, and relative entropy exponents ηw. We define and solve the amplitude Green functions recursively when the level of the fractal graph goes to infinity. We obtain exact recursive formulas for the amplitude Green functions, on which the hitting probabilities and expectation of the first-passage time are calculated, and using the recursive formula with the aid of Monte Carlo integration, we evaluate their numerical values. We also show that when the level of the fractal graph goes to infinity, with probability 1, the quantum random walks will return to the origin, i.e., the quantum walks on a Sierpinski gasket are recurrent.
Quantum walks on networks are a paradigmatic model in quantum information theory. Quantum-walk algorithms have been developed for various applications, including spatial-search problems, element-distinctness problems, and node-centrality analysis. Unlike their classical counterparts, the evolution of quantum walks is unitary, so they do not converge to a stationary distribution. However, for many applications, it is important to understand the long-time behavior of quantum walks and the impact of network structure on their evolution. In the present paper, we study localization of continuous-time quantum walks on networks. We derive an analytical expression for the long-time inverse participation ratio that depends on combinations of eigenvectors of the quantum-walk Hamiltonian. We then demonstrate how localization emerges in highly clustered networks that we construct by recursively attaching triangles. Building on the insights from this example, we then show that localization also occurs in Kleinberg navigable small-world networks and Holme-Kim power-law cluster networks. Our results illustrate that local clustering, which is a key structural feature of networks, can induce localization of quantum walks.